Testing apparatus utilizing position-force cross coupling control

ABSTRACT

Where a specimen, such as a wheel spindle of an automobile, is acted on by a first actuator to position the spindle along a vertical axis and a second actuator to apply a force or load along a horizontal axis, both of which actuators are pivotally mounted, cross coupling errors arise because the load being applied by the second actuator is influenced by the displacement produced by the first actuator. Compensation therefor is provided by multiplying a first signal representative of the change in horizontal displacement with respect to vertical displacement by a second signal representative of the vertical velocity of the specimen or the time rate of change of the vertical displacement thereof, the resulting product signal being employed in modifying the control of the horizontal actuator to provide the proper compensation.

73-6 9 sa GR 397180033 United State [111 3,718,033

Petersen A. [4 Feb. 27, 1973 TESTING APPARATUS UTILIZING S CTPOSITION'FORCE CROSS COUPLING Where a specimen, such as a wheel spindleof an au- CONTROL tomobile, is acted on by a first actuator to positionthe [75] Inventor; Niel R, P t n, Ho ki Mi spindle along a vertical axisand a second actuator to apply a force or load along a horizontal axis,both of which actuators are pivotally mounted, cross coupling errorsarise because the load being applied by the [22] Filed: April 20, 1971second actuator is influenced by the displacement [21] Appl. No; 135 560produced by the first actuator. Compensation therefor is provided bymultiplying a first signal representative of the change in horizontaldisplacement with respect [73] Assignee: MTS Systems Corporation,Minneapolis, Minn.

US. Cl. R to vertical displacement a econd signal repregenta- [5 Cltiveof the vertical velocity of the specimen o the [58] Field of Search..73/88 R, 71.7, 71.5 time rate f change of the vertical displacementthereof, the resulting product signal being employed in [56] ReferencesC'ted modifying the control of the horizontal actuator to UNITED STATESPATENTS provide the proper compensation.

2,799,l58 7/1957 Federspiel ..73/11 7 Claims, 1 Drawing Figure PrimaryExaminer-Jerry W. Myracle Attorney-Dugger, Peterson, Johnson & Westmanmass COUPL/l/G 3 38 34 30 I8 2 I/EfiT/Cfll ACTl/A me /4 Z 32 El 66OPERATING IN STROKE CONTROL 26 6 (Zom sown 40 44 f eas/755m 24 VAL VEGil/Al 68 l Mn/HER 7 [GAIN (aw/lam 28 ca/v rkaz z m STROKE COMO/F FDBK'40, nan/saucer pecan/w jZ. l

W y 70 CROSS (oupu/va SNAP/N6 NETWORK TESTING APPARATUS UTILIZINGPOSITION- FORCE CROSS COUPLING CONTROL BACKGROUND OF THE INVENTION 1.Field of the Invention The present invention relates generally totesting apparatus for simulating prescribed movements, and pertains moreparticularly to a system for use in compensating for cross couplingerrors.

2. Description of the Prior Art Cross coupling errors have provedparticularly troublesome in simulating road conditions experienced byautomobile wheel spindles in that such spindles traverse arcuate paths.Consequently, when a first actuator operating in position controlpositions the spindle vertically, a disturbing motion is imposed by thespindle on a second or horizontally disposed actuator operating in loadcontrol. Complex mechanical linkages have been devised to minimize thecross coupling effects. In this regard, patent application Ser. No.105,401 filed on Jan. 11, 1971 in the name of Thomas P. Lentz for AXLETEST DEVICE and which has been assigned to the present assignee. Thealluded to application has greatly simplified the mechanicalarrangements heretofore resorted to, doing so with bell cranks. Thepresent applicant in no way wishes to disparage the invention describedin the above-mentioned application. The present invention involves anelectrical solution for the cross coupling problem which possessescertain cost and flexibility advantages over the bell crank system orfurther enhances the overall fixture system performance.

SUMMARY OF THE INVENTION Accordingly, a general object of the presentinvention is to provide a simple and inexpensive control system forminimizing the load cross coupling errors that have in the past beenexperienced when a common reaction point follows either a linear ornonlinear path.

A more specific object is to reproduce the correct load and strokewaveforms required at a common tie point. In this regard, an aim of theinvention is to utilize the feedback signals provided by the system orother control media, altering one such signal with respect to the otherso that the testing closely approximates the command intelligenceembodied in both the control systems.

Yet another object of the invention is to provide a total servo controlsystem for two or more actuators which apply forces along at least twodifferent axes so that the actuator contained in the one servo controlchannel will be able to compensate for the disturbances caused byanother control channel.

Briefly, the invention comprises two (or more) actuators, one disposedvertically and the other (or others) horizontally, which act on a commonreaction point, such as the wheel spindle of an automobile. The verticalactuator exercises a position control function and the horizontalactuator (or actuators) perform a load control function. A bias signalis added into the load control servoloop that will force this slowerchannel to keep up with and reproduce the desired loads that are to beapplied to the spindle without adverse cross coupling effects. Amultiplier accepts a first input signal derived from either a linear ornonlinear, as the case may be, shaping network which denotes the changein horizontal displacement or movement with respect to the verticaldisplacement or movement. The first signal is multiplied by a secondsignal indicative of the vertical displacement with respect to time,that is vertical velocity, of the spindle. The product or output signalfrom the multiplier is in this way made representative of the horizontalvelocity that is required to impart the proper movement along both thevertical and horizontal axes so that a more accurate simulation results.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE denotes testingapparatus for simulating road movements experienced by a wheel spindlewith my control system diagrammatically superimposed thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although my invention can beused in other applications, it will find especial utility in the testingof automobile wheel spindles and therefore this type of testing has beenschematically portrayed in the accompanying drawing. Inasmuch as thetest specimen constitutes the wheel spindle of a conventionalfront-wheel suspension system of an automobile in the illustrativesituation, the suspension system need not be depicted in any detail.With this in mind a fragmentary portion of an automobile 10 has beenillustrated having a suspension system exemplified only by a simple coilspring 12. The spindle itself has been given the reference numeral 14,the spindle being an integral part of the suspension system 12.

In order to apply forces simulating a given set of road conditions, apair of actuators l6, 18 are employed. MTS Systems Corporation ofMinneapolis, Minn., the assignee of the invention herein described,manufactures an appropriate actuator under Model No. MTS Cylinder Series204. It will be discerned, however, that one actuator 16 is pivotallymounted at 20 to a fixed surface 22, such as the floor of a test cell orthe base plate of the test equipment. Somewhat similarly, the secondactuator 18 is pivotally attached at 24 to a fixed vertical surface 26,such as the wall of the test cell or a vertical panel associated withthe testing equipment. It will be appreciated that the pivotalconnections 20, 22 allow the actuators 16, 18 to swivel to the extentneeded in applying the requisite forces to the spindle 14. The actuator16 has its projecting piston rod 24 formed with a bearing 26 thatencircles the spindle l4. Inasmuch as the actuator 116 performs astroke-producing function, that is, it positions the spindle l4vertically, use is made of an internally mounted displacement transducer28, such as a linear differential transformer having a core movablewithin stationary exciting and pickup coils contained within the housingof the vertical actuator 16. The displacement transducer (LVDT) iseasily mounted inside the housing of an actuator having theabove-mentioned MTS model designation, and since this combination iscommercially available, it should suffice to merely state that thetransducer 28 provides a signal proportional to the stroke of the pistonrod 24 and hence in accordance with the position or displacement of thewheel spindle 14, which is the test specimen in the illustrative case,along a first or vertical axis.

Since the function of the actuator 18 is to apply the proper load orforce to the spindle 14 along a second or horizontal axis, its pistonrod 30 is adapted for connection to a load cell 32. Associated with theload cell is a bearing 34 that encircles the spindle 14 in the samemanner as the bearing 26.

It may be of some benefit to apply a straight arrow 36 indicative of thedirection in which the spindle displacement should occur vertically, anda curved arrow 38 representing the actual displacement that results dueto the constraint imposed by the actuator 18. More specifically, it willbe recognized that the swiveling caused by the pivoting of the actuator18 at 24 prevents a linear travel or stroke in the direction of thearrow 36 without some positional adjustment. The amount of verticaldisplacement in the direction of the arrow 36, which is generallytangential to the arcuate path denoted by the arrow 38, will beindicated by the letter y and later referred to by this designation,whereas the displacement in the horizontal direction, which is generallyradial or normal to the path 38, will be represented by the letter x. Itshould be appreciated at this stage of the description that thedifference between the position or location of the spindle 14 at a pointon the arcuate path 38 at a given moment and a point on the linear path36 is indicative of the cross coupling error. This is a dynamiccondition and this disturbance continually affects the dynamic loadbeing applied by the actuator 18. The present invention, however,continually corrects or compensates for this error as will soon becomemanifest.

Inasmuch as a two-channel command, one for position control and thesecond for load control, has been selected to exemplify the invention,it follows that two command signals are needed. Therefore, first andsecond program devices 40 and 42, which can be in the formof taperecorders, provide the proper command signals, the device 40 supplyingthe vertical program and the device 42 the horizontal program. Since inthe exemplary arrangement the spindle 14 is to be moved vertically, thestored signals furnished by the device 40 are such as to cause thedesired positioning of the spindle 14 along a vertical axis, whereas thesignals supplied by the device 42 cause the desired force or load to beapplied along a horizontal axis. As already explained, though, thevertical displacement of the spin dle 14 changes the actual loading sothat it does not correspond to the desired or commanded dynamic loading;the present invention, however, corrects for this.

As far as the program device 40 is concerned, this being the one that isto supply the vertical control signals, the signals therefrom aredelivered to a conventional controller 44 comprising a summing junctionor comparator 46 and a servo gain control 48 which connects with theinput of a valve amplifier 50. The output from the valve amplifier 50 isdelivered to a flow control servovalve 52 for supplying fluid to theactuator 16. A feedback conditioner 54 has its input connected to thetransducer 28 and its output to the second input of the summing junctionor comparator 46, the first input of the comparator 46 being connectedto the vertical program device 40. In this way, as is conventional, anydeviation between the command signal called for by the vertical programdevice 40 and the feedback signal from the conditioner 54 appears as anerror signal at the output of the comparator 46 and is used to controlthe servovalve 52 which in turn controls the vertical actuator 16.

A second controller 56 contains a summing junction or comparator 58 anda load servo gain control 60 which is connected through a summingjunction or comparator 62 and a valve amplifier 64 to a flow controlservovalve 66 associated with the actuator 18. A feedback signal is sentfrom the load cell 32 to the second input of the summing junction orcomparator 58 over a line containing a feedback conditioner 68, thecommand signal supplied by the horizontal program device 42 beingapplied to the other input of the summing junction or comparator 58.

What has been described up to this point with the exception of thecomparator 62 constitutes testing equipment that is conventional. Itwill be appreciated, though, that the vertical actuator 16 is includedin the position control channel and the horizontal actuator 18 in theload control channel. The common tie point represented by the spindle Mtraverses an arcuate path as denoted by the arrow 38. While the testspecimen has been designated as the spindle 14, it will be recognizedthat this connection could constitute any two degreebf-freedom system,the wheel spindle 14 being only an example of such a system. Hence, ifthe program device 40 produces a signal requiring a given amount ofvertical displacement to vertically position the spindle 14%, and thehorizontal program device 42 provides a signal that represents theamount of force or load that should be applied horizontally, then thetwo forces applied via the bearings 26 and 34 to the spindle M will beinterassociated.

As far as the specific test specimen in this illustrative situation isconcerned, it will be recognized that the vertical displacement y willbe considerably larger than the resulting horizontal displacement x.This is so by reason of the fact that the displacement y is indicativeof the actual bumps experienced in the simulated road test, whereas thedisplacement x is caused only by the arcuate constraint resulting fromthe pivotal mounting at 24 of the actuator 11%. It is desired, however,that a load indicative of horizontal forces picked up from the road beapplied to the spindle M by the actuator 18.

Hence, under dynamic conditions, the load control channel involving theactuator H8 cannot keep pace with the position control channel whichincludes the actuator 16. Therefore, a load error occurs due to thecross coupling effect. An aim of the invention is, of course, to providean effective, yet simple, solution to this vexing problem. Therefore, itmay be helpful at this stage to algebraically denote the correction thatis desired. In this regard, the desired correction is the velocity ofthe horizontal actuator 1% that is required in order to follow themovement produced by vertical actuator 16. This velocity, however, isinterdependent upon the vertical position and velocity. Therefore, thesituation can be algebraically presented as follows:

where dx/dt equals horizontal velocity and where dx/dy dy/dt equalsangle compensation times vertical velocity.

Having given the foregoing information, it is believed that my controlsystem denoted generally by the reference numeral 70 will be betterunderstood. From the drawing it will be seen that this system comprisesthose components appearing below the dashed line labeled 72; the system70 includes the summing junction or comparator 62 which has already beenreferred to. The control system 70 additionally includes a nonlinearshaping network 74 having its input connected directly to the outputside of the feedback conditioner S4 and therefore delivers into thenetwork 74 a signal representing the vertical motion or displacement y.Consequently, the output signal from the network 74 is representative ofthe rate of change as far as the horizontal displacement x is concernedwith respect to the vertical displacement y, even though realized from asimple potentiometer.

Where the horizontal displacement x is substantially proportional to thevertical displacement y, then the network 74 may constitute only asimple potentiometer with the wiper arm being set so as to produce anoutput representative of dx/dy on a proportional basis. Frequently,though, the relationship between x and y can become quite complex.Therefore, it will be well to place a nonlinear curve 76 within theblock designating the nonlinear shaping network 74. The hardware withinthe block 74 will thus be designed so as to represent, either preciselyor proximally, the particular nonlinear relation that prevails for agiven test. Stated somewhat differently, the benefits to be derived froma practicing of my invention can be readily understood irrespective ofthe content or design of the network 74.

My control system 70 further includes a differentiator 78 having itsinput also connected to the feedback amplifier 54 so that it likewisereceives a signal indicative of the amount of vertical displacement y.As its name implies, it differentiates this signal on a time basis sothat the signal at its output is representative of vertical velocity,more specifically the change in the vertical displacement y with respectto time.

Reference will now be made to a multiplier 80 having first and secondinputs 80a, 80b and an output 800. The multiplier 80 may be a multipliermanufactured by Motorola Semiconductor Products Inc. which carries themodel designation MC 1595. The multiplier 80 multiplies its two inputsignals (which are the output signals from the network 74 anddifferentiator 78) together and the product or output signal from theoutput 800, this being a signal having a value corresponding to thehorizontal velocity, is made available as a corrective signal for thesumming junction or comparator 62. However, it will be desirable toadjust the magnitude of the cross coupling compensation and this is donethrough the agency of a potentiometer 82, the output 800 being connectedto the resistance 82a and its wiper arm 82b being connected to thesecond input terminal of the summing junction or comparator 62. Thefeedback signal from the amplifier 68 being applied to the first inputof the comparator 62.

OPERATION Although the operation is believed obvious from the detaileddescription that has already been presented, nonetheless a brief summarymay be of assistance. In this regard, it will be appreciated that thespindle 14 traverses the arcuate path represented by the arrow 38. Thevertical displacement or motion produced by the actuator 16 whenoperating in this manner would influence the force applied by theactuator 18. The signal delivered to the actuator 18 is modified so asto take into consideration the vertical position of the spindle 14 atany moment, changing the actual applied load accordingly.

Consequently, as the actuator 16 moves the spindle 14 upwardly, thismotion being denoted by the arrow 36 (and the letter y), a signal inaccordance therewith is provided by the transducer 28 and delivered tothe input of the shaping network 74. In the exemplary situation thenetwork 74 may constitute a potentiometer, as already explained, andowing to the substantial proportional variation of x with respect to y,as can be discerned from the arrows 36 and 38, more specifically, thedivergence of the arrow 38 with respect to the arrow 36, it follows thatthe network 74 provides an output signal which is representative of thechange in the horizontal displacement x with respect to the verticaldisplacement y.

Simultaneously, the differentiator 78 provides a signal representing thechange in the vertical displacement y with respect to time, that isvelocity. By multiplying these two signals together with the multiplier80, an output signal which is the product of the two input signals isprovided, containing information which represents the change in thehorizontal displacement x with respect to time. This signal is inputtedto the summing junction 62 and is thus algebraically combined with thesignal furnished by the conditioner 56.

Consequently, the horizontal program signal from the device 42 ismodified by reason of the product signal from the multiplier 80 (andalso by the setting of the wiper arm 82b of the potentiometer 82) toreflect the change in the signal delivered to the flow controlservovalve 66. The resulting load or force applied to the spindle 14 bythe actuator 18 is closely representative of the load that should beapplied. The applied load, in this way, is not adversely affected by thecross coupling that would otherwise develop owing to the change inelevational position of the specimen 14.

It will be recognized that the entire system is exceedingly simple andthat it can be manufactured quite inexpensively. The effect of theexternal disturbance on the load control loop, which includes theactuator 18, the load cell 32, the feedback amplifier 68 and theconditioner 56, is minimized by merely generating a signal proportionalto the necessary horizontal velocity. This is achieved by using avelocity signal as one input to the multiplier 80. Thus, as far assimulating road conditions in the testing of automobiles, the use ofexpensive mechanical linkages is avoided, an electrical analogue of suchlinkages being provided instead to accurately duplicate what themechanical linkages have had to do in the past. This is achieved byincorporating signals from both the load and stroke control to producethe necessary velocity in the load control loop.

I claim:

1. A position-force cross coupling control system for a test specimenmounted for movement in at least one axis and mounted to receive a loadapplication in a different axis, including means to move said specimenalong said one axis and means to apply a load to the specimen along saiddifferent axis, the control system comprising means providing a firstsignal representa tive of the displacement of the specimen along saiddifferent axis with respect to its displacement along said one axis,means providing a second signal representative of the displacement ofsaid specimen with respect to time along said one axis, means formultiplying said first and second signals together to provide a productsignal, and means connected to said multiplying means for modifying theload applied to said specimen in accordance with said product signal tocompensate for changes in the load being applied due to the position ofthe specimen along said one axis.

2. Testing apparatus comprising first actuating means for moving a testspecimen along a first axis, second actuating means for applying a loadalong a second axis at an angle to said first axis, means providing afirst signal representative of the displacement of the specimen alongsaid second axis with respect to its displacement along said first axis,means providing a second signal representative of the velocity of saidspecimen along said first axis, a multiplier for multiplying said firstand second signals together to provide a product signal, and meansconnected between said multiplier and said second actuating means forcontrolling said second actuating means in accordance with the value ofsaid product signal to adjust the magnitude of the load being applied tocompensate for movement of said specimen along said first axis.

3. Test apparatus for providing a position control along a first axisand a load control along a second axis with respect to specimen having acommon reaction point comprising means to move said specimen along saidfirst axis and means to load the specimen along the second axis, firstmeans providing a signal representative of the amount of movement alongthe first axis, second means providing a signal representative of theamount of load applied along the second axis, a shaping networkconnected to said first means for providing a first signalrepresentative of the rate of change of the displacement of said secondmeans with respect to said first means, a differentiator connected tosaid first means for providing a second output signal representing theamount of displacement along said first axis with respect to time,multiplying means for multiplying said first and second signals togetherto provide a product signal representing the rate of change of thedisplacement along said second axis with respect to time, and means forproviding a control signal for said means to load said specimen whichcontains therein command information provided by said product signal andthe signal from said second means.

4. Test apparatus as defined in claim 3 in which a wheel spindleprovides said common reaction point, said apparatus including a firstpivotally mounted actuator for moving the spindle along the first axis,said means providing a signal representative of the amount of movementalong the first axis including a transducer connected to said shapingnetwork and to said differentiator, a first program device for producinga signal indicative of said spindle along said first axis, means forcomparing the signal from said first program device with the signal fromsaid transducer, said means providing a signal representative of theamount of load applied along the second axis including a load cell, asecond program device for producing a signal indicative of a desiredload to be applied to said specimen along said second axis, said meansfor providing a control signal for said means to load said specimenincluding a summing junction having first and second inputs and anoutput, said first input being connected to said load cell and saidsecond input to said multiplying means whereby the signal from saidsecond program device is modified by said product signal to change thecontrol signal for said means to load said specimen to compensate forthe change of position of said spindle along said first axis.

5. Test apparatus as defined in claim 4 including a cross couplingcompensation adjusting means connected between said multiplying meansand said summing junction.

6. A position-force cross coupling control system for a test specimenmounted for movement under load along a first axis including means formoving said specimen under load along said first axis, means comprisingpivotally mounted extendable and retractable load actuator coupled tothe specimen on a coupling pivot for applying load to the specimen alonga second axis at an angle to said first axis and which actuator normallymoves in an arc as the specimen moves along the first axis, the controlsystem comprising means providing a first signal representative of thedisplacement of said coupling pivot along said second axis with respectto its displacement along said first axis, means providing a secondsignal representative of the displacement of said specimen with respectto time along said first axis, means for multiplying said first andsecond signals together to provide a product signal, and means connectedto the multiplying means to modify the length of the actuator tocompensate for displacement of the specimen along said first axiswithout substantially changing the load applied to the specimen by saidactuator along said second axis.

7. The combination as specified in claim 6 wherein said actuatorcomprises a hydraulic actuator, and wherein said means connected to themultiplying means to modify the length of the actuator includes controlmeans for controlling the load applied by said actuator, signal meansfor determining the load applied by said actuator along said secondaxis, said means connected to the multiplying means also being connectedto said signal means.

w v w

1. A position-force cross coupling control system for a test specimenmounted for movement in at least one axis and mounted to receive a loadapplication in a different axis, including means to move said specimenalong said one axis and means to apply a load to the specimen along saiddifferent axis, the control system comprising means providing a firstsignal representative of the displacement of the specimen along saiddifferent axis with respect to its displacement along said one axis,means providing a second signal representative of the displacement ofsaid specimen with respect to time along said one axis, means formultiplying said first and second signals together to provide a productsignal, and means connected to said multiplying means for modifying theload applied to said specimen in accordance with said product signal tocompensate for changes in the load being applied due to the position ofthe specimen along said one axis.
 2. Testing apparatus comprising firstactuating means for moving a test specimen along a first axis, secondactuating means for applying a load along a second axis at an angle tosaid first axis, means providing a first signal representative of thedisplacement of the specimen along said second axis with respect to itsdisplacement along said first axis, means providing a second signalrepresentative of the velocity of said specimen along said first axis, amultiplier for multiplying said first and second signals together toprovide a product signal, and means connected between said multiplierand said second actuating means for controlling said second actuatingmeans in accordance with the value of said product signal to adjust themagnitude of the load being applied to compensate for movement of saidspecimen along said first axis.
 3. Test apparatus for providing aposition control along a first axis and a load control along a secondaxis with respect to specimen having a common reaction point comprisingmeans to move said specimen along said first axis and means to load thespecimen along the second axis, first means providing a signalrepresentative of the amount of movement along the first axis, secondmeans providing a signal representative of the amount of load appliedalong the second axis, a shaping network connected to said first meansfor providing a first signal representative of the rate of change of thedisplacement of said second means with respect to said first means, adifferentiator connected to said first means for providing a secondoutput signal representing the amount of displacement along said firstaxis with respect to time, multiplying means for multiplying said firstand second signals together to provide a product signal representing therate of change of the displacement along said second axis with respectto time, and means for providing a control signal for said means to loadsaid specimen which contains therein command information Provided bysaid product signal and the signal from said second means.
 4. Testapparatus as defined in claim 3 in which a wheel spindle provides saidcommon reaction point, said apparatus including a first pivotallymounted actuator for moving the spindle along the first axis, said meansproviding a signal representative of the amount of movement along thefirst axis including a transducer connected to said shaping network andto said differentiator, a first program device for producing a signalindicative of said spindle along said first axis, means for comparingthe signal from said first program device with the signal from saidtransducer, said means providing a signal representative of the amountof load applied along the second axis including a load cell, a secondprogram device for producing a signal indicative of a desired load to beapplied to said specimen along said second axis, said means forproviding a control signal for said means to load said specimenincluding a summing junction having first and second inputs and anoutput, said first input being connected to said load cell and saidsecond input to said multiplying means whereby the signal from saidsecond program device is modified by said product signal to change thecontrol signal for said means to load said specimen to compensate forthe change of position of said spindle along said first axis.
 5. Testapparatus as defined in claim 4 including a cross coupling compensationadjusting means connected between said multiplying means and saidsumming junction.
 6. A position-force cross coupling control system fora test specimen mounted for movement under load along a first axisincluding means for moving said specimen under load along said firstaxis, means comprising pivotally mounted extendable and retractable loadactuator coupled to the specimen on a coupling pivot for applying loadto the specimen along a second axis at an angle to said first axis andwhich actuator normally moves in an arc as the specimen moves along thefirst axis, the control system comprising means providing a first signalrepresentative of the displacement of said coupling pivot along saidsecond axis with respect to its displacement along said first axis,means providing a second signal representative of the displacement ofsaid specimen with respect to time along said first axis, means formultiplying said first and second signals together to provide a productsignal, and means connected to the multiplying means to modify thelength of the actuator to compensate for displacement of the specimenalong said first axis without substantially changing the load applied tothe specimen by said actuator along said second axis.
 7. The combinationas specified in claim 6 wherein said actuator comprises a hydraulicactuator, and wherein said means connected to the multiplying means tomodify the length of the actuator includes control means for controllingthe load applied by said actuator, signal means for determining the loadapplied by said actuator along said second axis, said means connected tothe multiplying means also being connected to said signal means.